Bizarre sea creatures illuminate the dawn of the animal kingdom

One of the greatest transformations in the history of life took place more than 600 million years ago, when a single-celled organism gave birth to the first animals. With their multicellular bodies, animals evolved into a dizzying array of shapes, such as whales weighing 200 tons, birds flying six miles into the air, and sidewinders gliding over desert dunes.

Scientists have long wondered what the first animals were like, including questions about their anatomy and how they found food. In a study published on Wednesday, scientists found tantalizing answers in a little-known group of gelatinous creatures called comb jellies. While the first animals remain a mystery, scientists discovered that comb jellyfish belong to the deepest branch of the animal family tree.

The debate over the origin of animals has been going on for decades. Initially, researchers largely relied on the fossil record for clues. The oldest definitive animal fossils date from approx 580 million years, although some researchers claimed to find even older ones. For example, in 2021, Elizabeth Turner, a Canadian paleontologist, reported a find 890 million year old fossils of possible sponges.

Sponges would make sense as the oldest animal. They are simple creatures, with no muscles or nervous system. They anchor themselves to the ocean floor, where they filter water through a maze of pores and trap bits of food.

In fact, sponges are so simple that it may come as a surprise that they are animals at all, but their molecular makeup reveals their kinship. They make certain proteins, such as collagen, that are only produced by animals. In addition, their DNA shows that they are more closely related to animals than to other life forms.

From the 1990s, when scientists collected DNA from more animal species, they tried to draw the animal family tree. In some studies, the sponges landed on the deepest branch of the tree. In this scenario, animals did not develop a nervous system until after the sponges branched off.

But in the early 2000s, other scientists came to a surprisingly different conclusion. They found that the deepest branch of animals were comb jellyfish – slender, oval creatures that often have a distinctive series of iridescent bands that flicker in the darkness of the deep ocean.

Many experts were reluctant to accept that conclusion because it meant animal evolution was stranger than they had realized. To begin with, comb jellies were not as simple as sponges. They have a nervous system: a web of neurons that circles their body controls their muscles.

To settle the comb-jelly versus sponge debate, researchers from around the world collected DNA from more species of ocean animals. And instead of looking at individual genes, researchers discovered how to sequence entire genomes.

But the avalanche of new data failed to settle the debate. Some scientists eventually put together a tree in which sponges were the deepest branch, while others got comb jellies.

The new study, published in the journal Nature, was based on a new method of using DNA to track animal evolution.

In previous studies, scientists looked at how certain mutations arise in different animal branches. A mutation can cause a single genetic letter, known as a base, to switch to a different letter. That mutation is then inherited by the offspring of an animal.

But these mutations can be unreliable markers of history. A base can switch from one letter to another, and millions of years later it can switch back to the original letter. Alternatively, the same base can switch to the same letter in two unrelated lines. This parallel evolution creates the illusion that the two genera are closely related.

In the new study, Darrin Schultz, an evolutionary biologist at the University of Vienna, and his colleagues looked instead at a different kind of genetic change. In rare cases, a huge piece of DNA is accidentally moved from one chromosome to another.

This massive mutation is less likely to mislead scientists. The chance of exactly the same piece of DNA going to exactly the same location a second time is astronomically small. It’s also nearly impossible for that piece to go back exactly where it came from.

“It’s direct evidence of something that happened,” said Dr. Schultz.

His team tracked the movements of genetic material in the chromosomes of nine animals, along with three unicellular animal relatives. They found a number of pieces of DNA in exactly the same place in the genomes of sponges and other animals. But these chunks were in a different position in comb jellies and single-celled animal relatives. That finding led Dr. Schultz and his colleagues concluded that comb jellies first split off from other animals.

“It’s a fresh look with a fresh approach to the question,” said Antonis Rokas, an evolutionary biologist at Vanderbilt University, who was not involved in the study.

In a 2021 study, Dr. Rokas and his colleagues also favor comb jellies. He said the new analysis was a strong confirmation.

“I’ve learned never to say the debate is over,” Dr. Rokas said. “But this moves the needle.”

The study raises intriguing new possibilities for what the common ancestor of living animals may have looked like. If comb jellyfish, with a nervous system and muscles, are the deepest branch of the animal tree, then perhaps early animals were not simple and spongy. They also had nervous systems and muscles. Only later did sponges release their nervous systems.

Dr. Schultz cautioned against thinking of comb jellies as living fossils, unchanged since the beginning of animals. “Something alive today cannot be the ancestor of something alive today,” he said.

Instead, researchers are now trying to comb jellies to see how similar and different their nervous systems are from those of other animals. Recently, Maike Kittelmann, a cell biologist at Oxford Brookes University, and her colleagues frozen comb jelly larvae so they could get a microscopic look at their nervous systems. What they saw left them stunned.

Throughout the animal kingdom, neurons are usually separated from each other by tiny gaps called synapses. They can communicate across the gap by releasing chemicals.

But when dr. Kittelmann and her colleagues began to inspect the comb jelly neurons, they struggled to find a synapse between the neurons. “At that point we thought, ‘This is curious,'” she said.

In the end, they couldn’t find any synapses between them. Instead, the comb jelly nervous system forms one continuous web.

When dr. Kittelmann and her colleagues reported their findings last month, they speculated yet another possibility for animal origins. Comb jellyfish may have evolved their own strange nervous systems independently of other animals, using some of the same building blocks.

Dr. Kittelmann and her colleagues are now inspecting other types of comb jellies to see if that idea holds up. But they won’t be surprised to be surprised again. “You don’t have to assume anything,” she said.